1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of semiconductor chips incorporated in a single semiconductor package.
2. Description of the Related Art
Demand for smaller sized semiconductor devices, and for smaller sized electronic apparatuses including the semiconductor device, is growing steadily. A conventional semiconductor device is formed by mounting a semiconductor chip on a lead frame and sealing the semiconductor chip with resin. An electronic apparatus includes a motherboard on which a plurality of semiconductor devices are mounted by leads. The size of the electronic apparatus becomes larger as number of semiconductor devices mounted on the motherboard increases.
In recent years, a semiconductor device having a CSP (chip size package) structure has been developed. In the CSP structure, a semiconductor chip is mounted on a substrate called an interposer, the pads of the semiconductor chip are electrically connected to the circuit of the interposer by wires or balls, and the semiconductor chip is sealed with resin. Balls constituting external terminals are arranged on the lower surface of the interposer, and the semiconductor device is mounted on the motherboard by coupling the balls to the motherboard.
The semiconductor device having a CSP structure can be formed in a very small size and therefore the electronic apparatus can also be formed in a very small size. Further, a semiconductor device, using an MCM (multichip module) structure and with a plurality of semiconductor chips arranged in a single package, has been developed and this can reduce the size of the electronic apparatus even more. In a semiconductor device having an MCM structure, the pads of several semiconductor chips are electrically connected to the circuit of the substrate called the interposer by wires or balls, and the balls constituting external terminals arranged on the lower surface of the interposer are connected to the motherboard. In this way, the number of elements mounted on the motherboard can be reduced, and therefore the size of the electronic apparatus can be further reduced.
Also, the semiconductor device generates heat, and it is required to dissipate this heat. Japanese Unexamined Patent Publication (Kokai) No. 5-74991 discloses a semiconductor device comprising a heat radiation lead arranged in the package of the semiconductor device and extended outside the package. Japanese Unexamined Patent Publication (Kokai) No. 8-130292 discloses a stack structure of semiconductor devices comprising a plurality of semiconductor devices put one on another in a stack, and electric connectors arranged on either side of the stack of the semiconductor devices, leads extending from the ends of each semiconductor device to the electric connectors on the two sides, and a heat radiation plate arranged between adjacent two semiconductor devices to improve the heat radiation characteristic of the stack structure of the semiconductor devices.
An MCM structure of semiconductor devices can be classified into a structure in which a plurality of semiconductor devices are arranged in parallel to the surface of a substrate, and a stack structure in which a plurality of semiconductor devices are stacked vertically on the substrate. With the stack structure, in spite of the presence of a plurality of semiconductor chips, the size of the semiconductor device (as viewed from above the semiconductor device) can be reduced to a size substantially equal to a size of a semiconductor device containing a single semiconductor chip.
However, in a semiconductor device having a stack structure such as, for example, a semiconductor device having two semiconductor chips stacked vertically, it is necessary to arrange that a first semiconductor chip is mounted on an interposer, and after electrically connecting the pads of the first chip to the pads of the interposer, the second chip is mounted on the first semiconductor chip, and then the pads of the second semiconductor are electrically connected to the circuit of the interposer.
For electrical connection of the pads, bumps (solder, gold, etc.) or bonding wires are used. The connection by bumps, however, poses not only the problem of an increased processes for bump fabrication and reliability in the jointed portions, but also the problem that it difficult to connect the pads of the second semiconductor to the circuit of the interposer, since the first semiconductor chip exists between the second semiconductor chip and the interposer.
The connection by wire bonding is free of the problems described above and has the advantage that the structure can be obtained comparatively easily and assuredly. Nevertheless, the problem of wire bonding is that it is not applicable to the case in which the first semiconductor chip and the second semiconductor chip have the same size. If, for example, a first semiconductor chip is mounted on the interposer, and the pads of the first chip is electrically connected to the interposer by wire bonding, after which a second chip is mounted on the first semiconductor chip, the second chip cannot be mounted snugly on the first semiconductor chip because the pads of the first chip are bonded by wires.
Further, some types of semiconductor chips accommodated in the semiconductor device operate at high frequencies and with a high output, and such a semiconductor device generates a considerable amount of heat in operation. Therefore, improved heat removal, for the operating semiconductor device, is also required.
An object of the present invention is to provide a semiconductor device, comprising a plurality of semiconductor chips accommodated in a single package in a stack, without increasing the size of the package and in which the semiconductor chips can be easily electrically connected within the package.
Another object of the invention is to provide a semiconductor device comprising a plurality of semiconductor chips accommodated in a single package and having an improved heat removal characteristic.
According to the present invention, there is provided a semiconductor device comprising; a substrate having pads, a first semiconductor chip mounted on the substrate and having pads, a plate member arranged on the first semiconductor chip and having an end at an inward position of the first semiconductor chip from the pads, a second semiconductor chip arranged on said plate member and having pads, a structure electrically connecting the pads of the first semiconductor chip and the pads of the second semiconductor chip to the pads of the substrate, and a seal member sealing the first semiconductor chip and the second semiconductor chip. The plate member can be a metal plate or a substrate having a circuit.
In the configuration described above, the. plate member having the end located inward of the first semiconductor chip from the pads is arranged on the first semiconductor chip, and the second semiconductor chip is arranged on the plate member. The second semiconductor chip can be arranged, in alignment with the first semiconductor chip, over the first semiconductor chip. The plate member is interposed between the first semiconductor chip and the second semiconductor chip at a position behind the ends of the first semiconductor chip and the second semiconductor chip. Even in the case where the pads of the first semiconductor chip are bonded by wires to the pads of the substrate (interposer), therefore, the second semiconductor chip can be arranged over the first semiconductor chip without interfering with the bonding wires. After the second semiconductor chip is arranged on the plate member, the pads of the second semiconductor chip can be connected to the circuit by wire bonding.
Preferably, the sides of the plate member are exposed to the outside from the surface of the seal member. By doing so, the heat generated in the semiconductor device is removed from the semiconductor device through the plate member. In the case where the plate member is configured of a stack of layers, moisture in the semiconductor device is released out of the semiconductor device through a gap between the plurality of the layers. As a result, a malfunction of the semiconductor device which otherwise might be caused by the water which may stay and evaporate in the semiconductor device and expand the seal member, is prevented.
Preferably, the plate member includes pads, and the structure electrically connecting the pads of the first semiconductor chip and the pads of the second semiconductor chip includes members electrically connecting at least one of the pads of the first semiconductor chip and the pads of the second semiconductor chip to the pads of the plate member and members for electrically connecting the pads of the plate member to the circuit of the substrate. For example, the pads of the second semiconductor chip are connected to the circuit of the substrate through the circuit of the plate member. By doing so, the points of electric connection can be rearranged.